Development of a 4D-MRI Technique for MRgRT

Purpose: To develop a 4D-MRI technique tailored for the daily patient setup verification and offline assessment of dynamic delivery for MR-guided adaptive radiotherapy.

Methods: The method consists of a sequential acquisition of 2D-cine data at neighbouring locations covering a certain volume of interest. Subsequently, 4D data is generated by sorting and binning the images as a function of their organ motion phase (e.g., due to breathing). The phantom and patient data was acquired on a 3T Siemens Verio a 2D-cine turbo flash with a flip-back pulse sequence. The image sorting was performed using a sinogram generated by subsequently collapsing each image in the 2D-cine series into one pixel line along the direction of organ motion and stacking these pixel lines into a 2D image map. The phase 3D volumes were generated by synchronizing the information resolved at each 2D-cine slice location. The 4D-MRI technique was used to assess offline the accuracy of VMAT delivery. This was done by reformatting the MR data to generate BEV projections for each VMAT segment (spatial location and timing) and subsequently evaluate the dose accumulation maps in the target and adjacent treated volume. A 4D-MRI-based workflow was designed for liver SBRT on a linac-MR on rails MRgRT platform, which is under development at our institution.

Results: The 4D-MRI technique showed very good performance when benchmarked against 4D-CT, 4D-CBCT, and 2D-cine MR phantom and patient liver data. The technique was used to assess the VMAT delivery for multiple scenarios considering different target treatment margins, including the case of real-time tracking.

Conclusions: The proposed 4D-MRI technique is a feasible approach for the assessment of daily pre-treatment target location and motion on an MRgRT platform consisting of an MRI scanner (goal: imaging) moving on rails to interact with a linac (goal: deliver treatment) via a robotic patient table.